Sailing downwind in a cruising cat is not much different than any other sailboat. In this post two conditions, one of heavy wind 20 plus knots and one of light wind say 10 knots or less, are explained.
When the wind is strong, 20 knots plus, it’s usually much safer to “heat up” or head up a bit to prevent an accidental jibe or wrap your spinnaker around the forestay. When you’re sailing dead downwind in stronger winds it’s wise to use a preventer hooked to the main boom should an accidental jibe occur. While using a spinnaker it’s not wise to sail dead downwind as when you catch a wave and surf down the face, you increase boat speed and decrease the apparent wind on the spinnaker… often times so much that the apparent wind may go to less than 5 knots. In this case the spinnaker may easily collapse and potentially wrap around the forestay. Some Cat sailors will argue this and prefer sailing under spinnaker alone without the mainsail. Although this is easy, low maintenance downwind sailing, maybe for days at a time, I personally do not like to do this as the mainsail will be your friend in an unexpected squall or wind increase and you need to get the spinnaker down. With the main up it’s very easy to sail dead down wind and blanket or collapse the spinnaker behind the main if you need to do a quick take down. Especially if you’re short handed on crew.
In a strong squall you may not want any sails up at all and just bare pole directly downwind. I personally prefer having the mainsail up even if it’s reefed to its minimum size in order to maintain steerage of the boat. Sailing wing and wing with a jib or genoa in a strong breeze is usually a very good option dead down wind as long as a preventer is used. Try using a spinnaker sheet attached to your jib or Genoa as the lead is usually further outboard and will help keep the wing and wing full. If you’re using a spinnaker be sure to head up a bit from dead down wind as this will keep the spinnaker with cleaner wind and less apt collapse as you surf down the waves. In large waves, be conservative and shorten sail and try not to sail faster than the speed of the waves otherwise you run the risk of surfing down the wave and burying your bow into the next wave. Generally not a good practice in cats. You slow abruptly as you hit the wave ahead and load up your rig or worse sink the bows and pitch pole. Not good!
As the wind lightens up say less than 12 knots, spinnaker or not, it’s much faster to to head up to a broad reach and increase your apparent wind. Cruising Cats can be heavy and they want to sail on a broad reach downwind. Forget about sailing dead downwind with a spinnaker up in lighter wind. You can sail wing and wing with a jib or Genoa dead down wind provided you’re in no rush.
As you head up to a broader angle or broad reach, you then have to be thinking about downwind jibe angles. If your waypoint or destination is downwind, eventually you will have to jibe to get there. Here is the easiest way to determine your jibe angle: In lighter winds less than 10 knots, it’s easy, in a cruising cat it’s usually 90-100 degrees. In other words, when your mark, waypoint or say a Harbor entrance bears 90 degrees off your current compass heading it’s time to jibe. This is true under spinnaker or any foresail. As the wind freshens it’s another story as 90 degree angles will be to much with a spinnaker and you will run the risk of over stressing the sail and destroying it. The angle should be closer to 50-60 degrees off your current heading.
Here is the easiest way to determine your jibe angle as the wind freshens. Sailing along, what is the true wind angle off your transom. What is the angle of the waves off your transom? Simply put, after you jibe, you want the wind angle (wave angle) off your transom to be the same as before you jibed. Look at the direction of the waves off your transom. What is the angle of the waves off your transom. Are they at a 45 degree angle? If so, your jibe angle is double that or 90 degrees. Is the wind strong and the angle of the waves only 10 degrees off your transom? Than your jibe angle would be only 20 degrees off your current course. To be more exacting and mathematical of your jibe angles, try standing over the top of your compass. What does the aft lubber line on the compass read? Look into the compass angle of the waves. Better yet, use a hand bearing compass. What is the difference between the two bearings? 10,20,30,45,50? Whatever it is, double it and that’s your jibe angle.
In the Northern hemisphere, if you’re jibing from Starboard tack to port or turning the boat to port in order to jibe than you would subtract your current compass course. Conversely, if jibing from port tack to Starboard or turning the boat to Starboard in order to jibe than you would add the jibe angle to your current course. In both cases, this will be your new compass course after you jibe. Most commonly sailing a Cat in moderate breeze, the waves or true wind angle will be approximately 30 degrees off the transom making the jibe angle 60 degrees. Important to remember a 90-100 degree jibe angle is for light winds only! Don’t risk damaging your beautiful spinnaker.
Does your new compass course after you jibe take you to where you want to go? If so, you did it right. If not, it’s no big deal. You will either have to jibe again or if you went to far on the previous jibe and your new course apparent wind angle is further forward than you expected, you may want to temporarily head off the wind and take down your spinnaker before you damage it.
In summary, cats will sail better downwind in lighter to moderate winds by heading up from dead down wind “heating it up.” In stronger winds, be very careful sailing dead down wind. Use a mainsail preventer and be careful surfing down the waves and collapsing the spinnaker and potentially wrapping it on the forestay. Or better yet, if you must sail dead down wind, take the spinnaker down and wing and wing the foresail.